Electron microscope having a plurality of coaxial cryogenically cooled lenses

ABSTRACT

The present invention relates to a lens system for particle radiation devices, particularly electron microscopes, which includes means for adjusting the particle beam, a plurality of superconducting cryogenically cooled magnetic lenses, a cryostat in which the lenses are received and a central supporting tube in the cryostat on which the lenses are mounted. An electron microscope produced according to the present invention is substantially free from thermal drift phenomena, is substantially insensitive to mechanical vibrations, can be readily protected against the influence of external magnetic fields, and can be assembled and disassembled conveniently for cleaning or exchanging parts.

' United States Patent Herrmann et al.

ELECTRON MICROSCOPE HAVING A PLURALITY OF COAXIAL CRYOGENICALLY COOLEDLENSES Inventors: Karl-Heinz Herrmann, Berlin;

Reinhard Weyl, Assling; Helmut Zerbst; Isolde Dietrich, both of Munich,all of Germany Siemens Aktiengesellschaft, Berlin & Munich, GermanyFiled: Feb. 7, 1974 Appl. No.: 440,332

Assignee:

Foreign Application Priority Data Feb. 16, 1973 Germany 2307822 US. Cl.250/396; 250/311 IntJCl. G21K 1/08 Field of Search 250/310, 311, 396,397, 250/398, 399

References Cited UNITED STATES PATENTS ll/196l Buchhold 250/396 1 Oct.28, 1975 Dietrich et a1 250/396 Dietrich et a] 250/396 PrimaryExaminer-Davis L. Willis Attorney, Agent, or Firm-Hill, Gross, Simpson,Van Santen, Steadman, Chiara & Simpson [57] ABSTRACT The presentinvention relates to a lens system for particle radiation devices,particularly electron micro? scopes, which includes means for adjustingthe particle beam, a plurality of superconducting cryogenically cooledmagnetic lenses, a cryostat in whichthe lenses are received and acentral supporting tube inthe cryostat on which the lenses are mounted.An electron microscope produced according to the present invention issubstantially free from thermal drift phenomena, is substantiallyinsensitive to mechanical vibrations, can be readily protected againstthe influence of external magnetic fields, and can be assembled'anddisassembled conveniently for cleaning or exchanging parts.

24 Claims, 6 Drawing Figures 1 Sheet 1 of 4 3,916,201 I US. Patent Oct.28, 1975 US. Patent 0ct.28,1975 sheetzom 3,916,201

Fig.2

US. Patent Oct. 28, 1975 Fig.3

Sheet 3 of 4 U.S. Patfi'lt Oct. 28, 1975 Sheet4 o f4 3,916,201

Fig.6 2009i! ill ZUUL ELECTRON MICROSCOPE HAVING, A PLURALITY OF COAXIALCRYOGENICALLY COOLEI) LENSES BACKGROUND OF THE INVENTION 1. Field of theInvention This invention is in the field of lens systems for particleradiation applications, particularly electron microscopes involving theuse of cryogenically cooled magnetic lenses which are received in acryostat and are supported on a central support tube located in thecryostat.

2. Description of the Prior Art The prior art has described columns foran electron microscope with superconducting lenses, including aplurality of condenser lenses, intermediate lenses, an objective lensand a projective lens. Each individual superconducting lens is arrangedin a helium cryostat which can be displaced horizontally on a platformin order to enable alignment of individual lenses in relation to thecolumn axis and the objective lens. This prior art arrangement is notonly relatively elaborate but is also sensitive to thermal driftphenomena. It is also difficult to dismantle. Dismantling of this typeof column is necessary if the internal components such as the polepieces and the diaphragms are to be cleaned or replaced.

SUMMARY OF THE INVENTION The present invention provides a column for anelectron microscope or the like using superconducting lenses, thestructure being such that it is substantially free of thermal driftphenomena, is largely insensitive to mechanical vibrations, can readilybe protected against the influence of externalmagnetic fields such asalternating current fields, and whose individual components which arelocated in proximity to the electron beam can be readily cleaned and/orexchanged in relatively simple fashion. The present invention alsoprovides a structure which is substantially more compact than prior artstructures.

The lens system of the present invention utilizes a plurality ofsuperconducting magnetic lenses with means for magnetic adjustment ofthe particle beam. The lenses of the new systemare arranged in acryostat and are assembled on a central supporting tube located in thecryostat. In the simplest device, the system consists of the objective'lens and an intermediate lens which is used for secondary magnification.In the preferred form of the invention, two or more lens systemsdesigned in accordance with the present invention are provided whichform component parts of an overalllens system, namely, the overallelectron microscope. '55

Inside the support tube, there are diaphragms and/or pairs of polepieces and/or adjusting systems for adjusting the axis of the particlebeam.

It is a relatively simple matter to clean a lens system in accordancewith the present invention or to exchange parts inside the system,particularly in that form of the invention in which there is a sleeve ofelectrically conductive material provided inside the'support tube, thesleeve being designed to be removable from the support tube but fittingrelatively closely inside the q tube. Components which are arrangedinside the support tube are located inside the sleeve for readyremovability.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects, features and advantagesof the inven tion will be readily apparent from the followingdescription of certain preferred embodiments thereof, taken inconjunction with the accompanying drawings, although'variations andmodifications may be effected without departing from the spirit andscope of the novel concepts of the disclosure, and in which:

FIG. 1 is a view partly in elevation and partly in crosssection of anelectron microscope assembly according present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS I v FIG. 1 illustrates a column1 of an electron microscope employing superconducting lenses. A cryostathousing is provided in which the lens system components, to be describedsubsequently, are located and into which liquid helium is introduced inorder to cool the components. Housing sections 2, 4 and 6aresuperimposed together to form a closed cryostat housing with anopening 8 for the introducion of'liquid helium and with an opening 9 forgaseous helium and for the passage of electrical leads into the interiorof the cryostat housing. Reference numerals 3 and 5 have beenapplied toflanges through which the sections 2, 4 and 6 for the passage of liquidhelium through the complete cryostat housing.

Reference numeral 104 has been applied to a plate which contains a boresealed in vacuum'tight relationship for the introduction 'of thespecimen. The bore 105, as seen in FIG. I, is horizontally disposed. The

meral 106, servingfor the passage of the helium through the interior ofthe cryostat housing.

Condenser lenses have been identified at numerals 11, 12 and 13, and theintermediate lenses at numerals l5, l6 and 17. The objective lensbecause of the presf ence of the plate 104 is split into two parts 14and 114 located respectively above and below the plate 104 and togetherforming a lens. In the embodiment of FIG. 1,

the housing, there is a support tube 24 to which the lens portions 14and 1 l4 and the parts of the first intermediate lens 15 are attached.Inside section 6 of the housing I there is a support tube 26 upon whichthe lenses 16, 17

and 18 are arranged, lens 18 being a projective lens.

In accordance with the preferred embodiment of the present invention,the support tubes 22, 24 and 26 have individual sleeves 23, 25 and 27fitted into them. These sleeves are provided without any substantialclearance between the outer diameter of the sleeve and the innerdiameter of the tube, but the sleeves are nevertheless convenientlyremovable from the support tubes.

Inside the sleeve 23, for each of the lenses 11, 12 and 13, a pair ofpole pieces 31, 32, and 33, respectively, are provided, the pole piecesbeing made of ferromagnetic material. Preferably, these pairs of polepieces are arranged to be removable from the interior of the sleeves sothat they can be individually cleaned. In addition to the pairs of polepieces, there are a number of diaphragms 41, 141, 42 and 43 inside thesleeve 23 which are used to stop out the electron beam. Thesediaphragms, in addition, are preferably made to be removable. Theattachment and centering of the diaphragms and pole pieces can beeffected by means of adjuster mechanisms which are operated followingremoval. Pole pieces can also be arranged inside the tube, in the mannershown with respect to the lens 15.

Inside the sleeve 27 in the support tube 26, there are pairs of polepieces 36 and 37 as well as diaphragms 46 and 47. Inside the sleeve 25which is inside the support tube 24 of the housing, there is a furtherdiaphragm 45 and screening elements 46 and 47 composed ofsuperconductive material. These screening elements are used to shape themagnetic focusing field of the screening lens operating as the objectivelens.

Because the sleeves 23 and 27 can be quite readily removed, the sleeve25 which is located inside the support tube 24 of the section 4 can alsobe readily extracted from the overall lens system. In the focusing zoneof the objective lens 14, 114, that is, in the neighborhood of thescreening elements 46 and 47, it is important to insure that the wallsare free of any contaminating foreign bodies which could developelectrical charges and thus give rise to an unwanted superimposition ofan electric field.

The structure of the lens systems using support tubes in accordance withthe present invention, and particularly in the matter of removablesleeves, also facilitates the exchange of individual parts which forelectrooptical reasons are subjected to modification or wear, forexample, the consequence of electron impact.

Non-mechanical adjusting systems can be provided in the column tocorrect errors which may arise. In FIG. 1, reference numerals 51, 52,53, 54, 55, 56 and 57 are applied to adjusting systems of this kindarranged between the individual lenses and constituting deflectionsystems by means of which the electron beam can be deflected laterallyin any direction, and/or stigmators by means of which deviations on thepart of the lens fields from rotational symmetry can be corrected.

The adjusting systems 51, 52, 53 and 54 are located inside the supporttube 22 and inside the slide-in sleeve 23. Each individual adjustingsystem can consist, for example, of four individual coils arranged inone plane at right angles. Using the four coils, three of which areshown in the figure, a magnetic deflecting field can be generated bymeans of which an electron beam which is deviating from the axis can bedeflected back to the system axis.

The deflection systems referred to previously can, as mentioned, bearranged inside the support tube or the sleeve or outside the supporttube. The latter arrangement has been illustrated with respect to theadjusting system 58 in the direct vicinity of the objective lens.Adjusting systems can readily be positioned outside the support tube ifthe tube has a relatively small diameter. A small internal diameter isconvenient particularly for the support tube 24 of the lens system,consisting of the objective lens 14, 114, and the first secondarymagnification lens 15, because the geometric dimensions of the objectivelens in the neighborhood of the specimen are involved in the imagingproperties. Because of the small internal diameter of the tube 24, thepairs of pole pieces of the intermediate lens 15 are also outside thetube.

Using the structure of the present invention, a column with a largenumber of condenser lenses and a large number of additional secondarymagnification lenses can be produced. Because of the low installedheight and the large refractive power of superconducting lenses, theinstalled length of the column in accordance with the invention can bekept very short, that is, a very precise, stable and vibration resistantstructure is provided.

Using the principals of the present invention, it is possible to build acolumn for a beam voltage of3 megavolts, which has at least twocondenser lenses and three intermediate lenses, giving a magnificationof about 1,000,000 times, the length of the column being less than 1meter. Comparable known systems with non-superconducting lenses, on theother hand, have lengths up to 5 meters.

To shield the individual lens systems and therefore the overall lenssystem against external magnetic fields, in particular, alternatingcurrent fields, it is advisable to provide superconducting shieldingcylinders inside the cryostat housing. A shielding cylinder 202 of thiskind has been shown in FIG. 1 located in housing section 2.Corresponding shielding cylinders can also be arranged in the otherhousing sections. They are located in the liquid helium and aretherefore superconducting.

In connection with lenses 11 and 12, there has been shown an arrangementfor introducing the electric current flowing through the magneticwinding of the lens. The other lenses can be electrically connected in asimilar fashion. A single current supply unit can be employed toenergize different lenses or correcting systems, successively. Leads 212and 218 can be taken out of the interior of the cryostat through anopening such as the opening 9. Also inside the cryostat is asuperconducting short-circuiting line 214 or 219 which produces shortcircuits, respectively, between the lines 212 and 213 or 213 and 218,respectively. However, this short circuit is absent as long as the flowof current through the winding 215 surrounding the line 214 or thewinding 220 surrounding the line 219 produces sufficient heat for theline 214 or 219 to retain a normally conducting condition. Acontrollable generator 216 is provided for generating current and byoperation of switches 221 and 222, the current flow is either throughthe winding 215 or through the winding 220. While the short circuit 214is inoperative as a consequence of the heat developed in the winding 215with normal conduction occurring, the generator 217 feeds current intothe winding of the lens 11 independently of the load. The coil of lens12 is not energized because the current flow is through the shortcircuit line 219. As soon as the current in the lens 11 has built up,the current in the winding 215 is interrupted and the short circuit 214becomes operative so that a permanent superconduction current flows inthe winding of:the lens 11. Subsequently, in the same fashion, thecurrent can be introduced into thecoil 12; I r

A support tube used in accordance with the present invention can be madeup in sections of different material. The material of the support tubemay be superconducting, ferromagnetic or neither superconducting norferromagnetic in a given section. If a sleeveis provided inside asupport tube, the same applies to the sleeve. Which material is used atwhich point in the support tube and in the sleeve depends upon whether,at the particular location, ferromagnetic directing of magnetic lines offorce or screening of a magnetic field using the phenomenon ofsuperconduction is required, is acceptable or has to be avoided. Forexample, in the case of pole pieces which are arranged inside thesupport tube or the sleeve, it is necessary that between the pole piecesand the magnetic surround of the lens, for example, between 31 and 211,the magnetic lines of force should close. Consequently, the portions ofthe support tube 22 and the sleeve 23 in proximity to the ferromagneticsurround 211 on the lens 11 are ferromagnetic in nature so that thelines of force passing through 211 can cross to the pole piece 31without substantial difficulty. On the other hand, the support tube andsleeve at the location of the gap between the pole pieces of the pair 31should be non-ferromagnetic because otherwise there would be more orless a short circuit in respect to the particular magnetic fielddesigned to develop between the pole pieces of the pair 31 in theneighborhood of the axis of the lens system. In the neighborhood of theobjective lens 14, 114, the support tube 24 and the sleeve 25 must besuperconducting in the region of the screening elements 46, 47. Thissuperconducting condition is necessary so that no gap is created of thekind through which the lines of force could penetrate between thesuperconducting screen 2140 of the lens 14 and the screening element 46.On the other hand, in the lens gap between the section 14 and thesection 1 14, the magnetic field must enter the center of the lens inorder to exert a focusing action there. The support tube and sleevecannot therefore be superconducting or ferromagnetic in this zone. Inthe superconducting shielding lens 14, 114, the shielding elements 46and 47 act as far as focusing is concerned in the manner comparable withthe way the pairs of pole pieces act. In the neighborhood of the polepieces of the lens 15, the support tube and sleeve must beferromagnetic. However, between the pole pieces of the lens 15, thesupport tube and sleeve must be free of ferromagnetic or superconductingproperties. The support tube and sleeve can, however, be superconductingin some cases in the neighborhood of the pole pieces of the lens 15.

In the embodiment shown in FIG. 1, as mentioned, adjusting systems areprovided inside and outside the sleeve and support tube. Insofar asthese adjusting systerns, which consist of field coils, are locatedinternally, the sleeves and/or support tubes should be ferromagnetic inorder to produce an external annular pattern of closing of the lines offorce for the individual coils, for example, the adjusting system 51.Whenthe adjustof the axis of the column. It is convenient in the case ofexternal adjusting systems such as system 58 to provide for magneticreturn externally around the adjusting system, although this has notbeen shown in FIG. 1. If required, it is also possible to use aferromagnetic casing 2ll.of the lens as a return means.

The support tube and the sleeve are provided with bores as necessary orexpedient. For example, in the support tube 24 and the sleeve 25 thereare bores arranged in continuation of the bore 105 in the plate 104.Through the bore 105 and the associated bores in the support tube andsleeve, the specimen is introduced into the object plane of theobjective lens 14, 114.

FIG. 2 illustrates an embodiment of the invention in which a supporttube 522 passes centrally through the complete column. Inside thesupport tube 522 there is a through sleeve 523. Inside the sleeve 523,the lens structures of the type shown in FIG. 1 have been employed, andcorrespondingly numbered. In the same way as plate 104, a plate 1 104 isprovided in which horizontal bores 1105 and 1106 are formed. Throughthese bores slides 1110 and 1111 can be passed. These slides can be usedas the object mounts and/or as diaphragm mounts. The objective lensconsists of a lens split into two parts 504 and 514 and is provided witha superconducting winding. The two lens sections are surrounded by ahousing 516 of ferromagnetic material. Pole piece 534 is provided forthe objective lens.

The first intermediate lens 515 produces secondary I magnification andhas pole pieces 535 which are located inside the support tube 'andinside the sleeve 23. Support tube 522 and sleeve 523 at the locationswhere housing 516 and pole pieces 534 and 535 touch each other orapproach each other are made of ferromagnetic material. In theneighborhood of the gap between the individual pole pieces of the pairs534 and 535, the materials of the support tube and sleeve are notferromagnetic so that no magnetic short circuit can occur at thislocation.

FIG. 3 illustrates a further modified form of the invention including ahousing section 1304 which corresponds essentially to section 4 of theembodiment shown in FIG. 1, with a support tube 1224 with a stepped downconstruction directly below the objective lens. The objective lensconsists of the sections 604 and 614 and is of the type previouslydescribed in connection with FIGS. 1 and 2. In the embodiment of FIG. 3,a superconducting shielding lens is-provided with a superconductingshielding housing 1314 and with superconducting shielding elements1246and 1247. These cylindrical shielding elements surround the innerportion of the support tube 1224. The support tube 1224 has a diameteras small as possible so that the shielding elements 1246 and 1247 are inas close proximity as possible to the specimen or object, with theirmutually opposite edges forming the magnetic field, the specimen beinglocated at' the tip of the slide 1210 which is introduced through a bore1206 in the plate 1204. An opening 1206 is provided for the introductionof liquid helium.

Inside that portion of the support tube 1224 which has the largercross-section, there is a sleeve 1225 of ing system is arranged outsidethe support tube as, for

the type previously described in connection with the other embodiments.Inside the sleeve 1225 there is a diaphragm 1245. In the constructionshown in FIG. 3, the sleeve 1225 can only be removed from the supporttube 1224 from below, but this does not constitute any appreciablerestriction. The other details of the embodiment shown in FIG. 3correspond with the structure shown in FIG. 1, and the same referencenumerals have been applied.

FIG. 4 is a fragmentary view ofa further embodiment of the invention inwhich the shielding lens acts as an objective lens. In the embodiment ofFIG. 4, the shielding lens is split into two parts 704 and 714 betweenwhich a plate 1704 is located, the latter corresponding essentially tothe plates 1104 and 1204. Superconducting shielding elements 716 and 717are also provided, and the coils have been identified at referencenumerals 718 and 719.

A support tube 724 is provided about which the casings 716 and 717 arearranged. Those parts of the support tube 724 indicated by referencenumerals 726 and 728 are made of superconducting material. In the areas704 and 714, the parts 730 and 731 which are arranged inside the supporttube 724 act as shielding elements as explained in connection with theembodiment shown in FIGS. 1 and 3. The sections 727 and 729 of thesupport tube 724 consist of a material which is neither superconductingnor ferromagnetic but does have good thermal conductivity. The materialof the sections 727 and 729 therefore has no influence upon the shapingof the magnetic field in that zone in which the shielding elements 726and 728 are located opposite each other. The sections 727 and 729 can,for example, be an integral part of the support tube 724 and the plate1704. The parts of the support tube 724 can preferably be integrallyattached by welding. A pair of rings 732 and 733 serve to cool thosesides of the shielding elements which are disposed toward the gap. Theyshould be made of material having good thermal conductivity which isneither ferromagnetic nor superconducting. In operation, the interior ofthe support tube 724 is evacuated for the passage of the electron beamand consequently the welds provided in this area should be made vacuumtight.

The embodiment of FIG. 5 illustrates a structure with pole pieces bothinside a support tube 822 and inside a sleeve 823. A housing 816surrounds a lens winding 818. The heavily cross-hatched sections 841 and842 of the support tube 822 and a sleeve 823 consist of ferromagneticmaterial so that closure of the magnetic field between the pole pieces834 and 835 and the housing 816 is possible. The section 843 of thesupport tube and the corresponding section of the sleeve is neitherferromagnetic nor superconducting so that the magnetic field which is tobe developed between the tips of the pole pieces 834 and 835 is notshort circuited.

FIG. 6 is a schematic illustration of an electron microscope with a lenssystem in accordance with the present invention. Only the cryostathousing 2000 of the lens system has been shown, this housingcorresponding to the sections 2, 4 and 6 of FIG. 1. A vacuum tightchamber 2002 contains the parts of the support tube or severalindividual support tubes, plus any extensions thereof in the upward ordownward directions. In the top end of the vacuum chamber 2002, there isan electron source 2003. This, for example, may be an incandescentfilament. A viewing chamber 2004 is provided at the bottom end of thevacuum tight chamber 2002. Inside the viewing chamber there is a viewingscreen for inspecting the enlarged electron microscope image. Theinterior of the vacuum tight chamber 2002 is connected to a pipe 2006 toa vacuum pump 2007.

The annular anode for the assembly is identified at reference numeral2008. A voltage source 2009 is used to maintain a voltage between theelectron source 2003 and the anode 2008. The broken line 2009 indicatesthe path of the electron beam inside the vacuum tight chamber throughthe lens system in accordance with the present invention.

It should be evident that various modifications can be made to thedescribed embodiments without departing from the scope of the presentinvention.

We claim as our invention:

1. A lens system for a beam of particle radiation comprising means foradjusting the particle beam, a plurality of superconductingcryogenically cooled magnetic lenses, a cryostat in which said lensesare received, and a central support tube in said cryostat on which saidlenses are mounted.

2. The system of claim 1 in which said magnetic lenses include anobjective lens and an intermediate lens having coinciding optical axes.

3. The lens system of claim 1 in which there is at least one diaphragmwithin said support tube.

4. The lens system according to claim 1, wherein said lens systemincludes at least one magnetic lens with ferromagnetic pole piecesarranged inside said support tube, said support tube in proximity tosaid pole pieces being composed at least partially of ferromagneticmaterial.

S. A lens system according to claim 1 in which shielding elements ofsuperconducting materials associated with at least one magnetic lens arelocated inside said support tube, and the support tube in the vincinityof said shielding elements is composed at least partially ofsuperconducting material.

6. A lens system according to claim 1 which includes at least oneadjusting system within said support tube.

7. A lens system according to claim 1 including at least one magneticlens with pole pieces located outside said support tube.

8. A lens system according to claim 1 including at least one magneticlens with shielding elements located outside said support tube.

9. A lens system according to claim 1 including at least one adjustingsystem located outside said support tube, said support tube beingneither ferromagnetic nor superconducting in proximity to said adjustingsystem.

10. A lens system according to claim 1 which includes a sleeve ofelectrically conductive material in said support tube, said sleeve beingclosely fit into said support tube but removable therefrom.

11. A lens system according to claim 10 in which a diaphragm ispositioned within said sleeve.

12. A lens system according to claim 10 in which a pair of pole piecesis positioned within said sleeve.

13. A lens system according to claim 10 in which an adjusting system ispositioned within said sleeve.

14. A lens system according to claim 1 including an objective lens, aplate in proximity to said objective lens, said plate containing atleast one radial bore for the introduction of a specimen, and containingat least one axially extending bore for the circulation of cool-- ant insaid cryostat.

15. A lens system according to claim 1 including several magnetic lensesconnected in series with respect to their energizing means.

16. A lens arrangement which includes a plurality of lenssystems insuperimposed relation, each of said lens systems comprising means foradjusting a particle beam, a plurality of superconducting cryogenicallycooled magnetic lenses, a cryostat in which said lenses are received,and a central support tube in said cryostat on which said lenses aremounted.

17. A lens arrangement according to claim 16 in which said lensarrangement includes a condenser lens system, a lens system consistingof an objective lens and a first intermediate lens, and a magnificationlens system including at least one intermediate lens and a projectivelens.

18. A lens arrangement according to claim 16 in which the individuallens systems are assembled so as to provide a single internal space forcoolant.

19. A lens arrangement according to claim 16 in which all the lenssystems are disposed in a single cryostat.

20. A lens arrangement according to claim 16 in which all the lenssystems are arranged on a single support tube.

21. The lens arrangement according to claim 16 which includesferromagnetic polepieces of at least one magnetic lens arranged insideof the support tube, the support tube in proximity to said pole piecesbeing composed at least partially of ferromagnetic material.

22. A lens arrangement according to claim 16 in which shielding elementsof superconducting materials associated with at least one magnetic lensare located inside said support tube, and the support tube in thevicinity of said shielding elements is composed at least partially ofsuperconducting material.

23. A lens arrangement according to claim 16 which includes at least oneadjusting system within said support tube.

-24. A lens arrangement according to claim 16 which includes a sleeve ofelectrically conductive material in said support tube, said sleeve beingcloselyfit into said

1. A lens system for a beam of particle radiation comprising means foradjusting the particle beam, a plurality of superconductingcryogenically cooled magnetic lenses, a cryostat in which said lensesare received, and a central support tube in said cryostat on which saidlenses are mounted.
 2. The system of claim 1 in which said magneticlenses include an objective lens and an intermediate lens havingcoinciding optical axes.
 3. The lens system of claim 1 in which there isat least one diaphragm within said support tube.
 4. The lens systemaccording to claim 1, wherein said lens system includes at least onemagnetic lens with ferromagnetic pole pieces arranged inside saidsupport tube, said support tube in proximity to said pole pieces beingcomposed at least partially of ferromagnetic material.
 5. A lens systemaccording to claim 1 in which shielding elements of superconductingmaterials associated with at least one magnetic lens are located insidesaid support tube, and the support tube in the vincinity of saidshielding elements is composed at least partially of superconductingmaterial.
 6. A lens system according to claim 1 which includes at leastone adjusting system within said support tube.
 7. A lens systemaccording to claim 1 including at least one magnetic lens with polepieces located outside said support tube.
 8. A lens system according toclaim 1 including at least one magnetic lens with shielding elementslocated outside said support tube.
 9. A lens system according to claim 1including at least one adjusting system located outside said supporttube, said support tube being neither ferromagnetic nor superconductingin proximity to said adjusting system.
 10. A lens system according toclaim 1 which includes a sleeve of electrically conductive material insaid support tube, said sleeve being closely fit into said support tubebut removable therefrom.
 11. A lens system according to claim 10 inwhich a diaphragm is positioned within said sleeve.
 12. A lens systemaccording to claim 10 in which a pair of pole pieces is positionedwithin said sleeve.
 13. A lens system according to cLaim 10 in which anadjusting system is positioned within said sleeve.
 14. A lens systemaccording to claim 1 including an objective lens, a plate in proximityto said objective lens, said plate containing at least one radial borefor the introduction of a specimen, and containing at least one axiallyextending bore for the circulation of coolant in said cryostat.
 15. Alens system according to claim 1 including several magnetic lensesconnected in series with respect to their energizing means.
 16. A lensarrangement which includes a plurality of lens systems in superimposedrelation, each of said lens systems comprising means for adjusting aparticle beam, a plurality of superconducting cryogenically cooledmagnetic lenses, a cryostat in which said lenses are received, and acentral support tube in said cryostat on which said lenses are mounted.17. A lens arrangement according to claim 16 in which said lensarrangement includes a condenser lens system, a lens system consistingof an objective lens and a first intermediate lens, and a magnificationlens system including at least one intermediate lens and a projectivelens.
 18. A lens arrangement according to claim 16 in which theindividual lens systems are assembled so as to provide a single internalspace for coolant.
 19. A lens arrangement according to claim 16 in whichall the lens systems are disposed in a single cryostat.
 20. A lensarrangement according to claim 16 in which all the lens systems arearranged on a single support tube.
 21. The lens arrangement according toclaim 16 which includes ferromagnetic polepieces of at least onemagnetic lens arranged inside of the support tube, the support tube inproximity to said pole pieces being composed at least partially offerromagnetic material.
 22. A lens arrangement according to claim 16 inwhich shielding elements of superconducting materials associated with atleast one magnetic lens are located inside said support tube, and thesupport tube in the vicinity of said shielding elements is composed atleast partially of superconducting material.
 23. A lens arrangementaccording to claim 16 which includes at least one adjusting systemwithin said support tube.
 24. A lens arrangement according to claim 16which includes a sleeve of electrically conductive material in saidsupport tube, said sleeve being closely fit into said support tube butremovable therefrom.